The invention relates to an apparatus for reading a disc-shaped record carrier with a spiral information track provided with optically readable digitally coded information, said digital information being recorded in the information track as a stream of data bits of constant spatial bit-frequency independently of the diameter of the track turn and comprising a sequence of detectable information blocks with a fixed number of data bit, such an apparatus is provided with a radiation source for emitting a read beam, an optical system for projecting said read beam as a read spot on the record carrier, a detection system for detecting the information contained in the read beam after cooperation with the record carrier, a conversion arrangement for converting said information into an electric information signal, a positioning system for controlling the radial position of the read spot on the record carrier, and a servo system for controlling the scanning speed of the record carrier in order to obtain an electric information signal of a desired bit frequency which is constant independently of the radial scanning position on the record carrier.
Currently, recording/reproducing systems employing optically readable disc-shaped record carriers are in the focus of interest. An example of systems of this type is the so-called Compact Disc Digital Audio System, as inter alia described in Philips Technical Review, Vol. 40, 1982, no. 6, which is incorporated by reference in this application.
This system employs a disc-shaped record carrier in which digitally coded audio information is recorded in a spiral information track in the form of an optically readable relief structure. Thus, the information track contains a stream of data bits representing the audio signal. These data bits are recorded in the information track with a constant spatial frequency, i.e. each data bit occupies a constant unit of length in the information track regardless of the radial position on the record carrier. This means that the number of data bits per track turn varies as a function of the radius of the track turn, i.e. the number of data bits per track turn increases as the radius increases.
When reading such a record carrier the information track is scanned by a read spot formed on the record carrier by a read beam, so that the data bits of the digital audio signal are read serially. The scanning speed, i.e. the relative speed between the record carrier and the read spot in the longitudinal direction of the information track is then controlled in such a way that the digitally coded audio signal is read with a constant bit frequency. For this purpose the speed of the record carrier is generally controlled depending on the bit frequency of the digital signal being read. When the spiral information track is scanned from the inside towards the outside, the rotational speed of the record carrier therefore decreases gradually.
For an optimum read-out of the record carrier various servo-systems which cooperate with the optical read system are required. For example, the read apparatus comprising a positioning system for controlling the radial position of the read spot on the record carrier so as to ensure that the read spot remains centred on the information track despite possible eccentricities of this information track. Furthermore, the read apparatus comprises a focusing system which ensures that the read beam remains correctly in focus on the information surface of the record carrier. Since the width of the information track is very small and the information density very high, stringent requirements are imposed on these servo systems.
Primarily, the servo systems must have a high gain and a large bandwidth in order to achieve an accurate positioning and focusing of the read spot. Such servo-systems, however, are very sensitive to spurious signals as a result of imperfections in the record carrier such as inclusions in the recording material and flaws in or contaminations of the record carrier surface. As a result of such imperfections, the information read-out may become disturbed for some time. However, the control signals applied to the servo-systems are also disturbed. As a result of the high gain and large bandwidth of these servo-systems, such a disturbance may result in one or both servo-systems overstepping their control ranges, so that in fact they are temporarily inoperative. Since a correct read-out is not possible until both servo-systems have locked-in again, this means that in effect the read-process may be disturbed for a substantially longer time than would be anticipated on account of the imperfection in the record carrier. An additional undesired factor is that the temporary failure of the radial positioning system may cause the read spot to be shifted over one or more track distances before this positioning system is again capable of keeping the read spot centred on the information track, which results in an extremely annoying audible transient in the reproduction of the recorded audio signal.
This susceptibility of the servo-systems to imperfections in the record carrier can be reduced by reducing the gain and bandwidth of the servo-systems. In practice a compromise is therefore made between these two conflicting requirements imposed on the gain and bandwidth of the two servo-systems. This compromise leads to an increased susceptibility of the apparatus to external shocks and vibrations. This is not objectionable if the apparatus is used in a stable environment, for example a living room. However, if the apparatus is to be used in a less stable environment, for example a car, this will pose a serious problem.